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Thursday, October 20, 2011

I could pick him out of a lineup anywhere. He had a big scar.

If you have a TV or a DVD player, chances are you have deep familiarity with the criminal justice system.Watching the police solve a difficult crime makes for good and suspenseful entertainment.And it is always so powerful to watch the victim of a crime identify the perpetrator from a lineup.That eyewitness identification also has a huge influence on juror’s beliefs about the guilt of a defendant on trial.

Because of the importance of these eyewitness identifications, there has been a lot of research on factors that affect the reliability of identifications from police lineups.

An interesting study of lineups by Theodora Zarkadi, Kimberly Wade, and Neil Stewart appeared in the December, 2009 issue of Psychological Science.This study looked at the best way to run a lineup when the perpetrator of the crime had a distinctive feature like a scar, a bruise, or a distinctive facial piercing.

When an eyewitness sees someone with a distinctive feature, there is a tendency for them to focus just on that feature.So, if the police pick up an innocent person who has a similar feature, the eyewitness is quite likely to mistakenly identify the innocent person on the basis of this feature.

It has become routine now to do lineups with photographs of suspects rather than with a live lineup.By using photographs, it is possible to use Photoshop to remove or add features to faces.This ability to alter pictures leads to two ways that police might change pictures to improve the accuracy of a lineup.

The most straightforward possibility would be to remove the distinctive feature from a suspect.If an eyewitness saw a scar, then the picture of the suspect could be shown without the scar.

An alternative would be to add the same distinctive feature to all of the faces in the lineup.That is, if the suspect is shown along with 5 other faces, the same scar could be added to all of the faces.

Which type of correction is more effective?

In one of the studies in this paper, people viewed 32 faces for two seconds each.Of these, six of the faces had a distinctive feature like piercings, bruises, or scars.(The distinctive features in this study were added to pictures of faces that did not originally have these features.)

After a delay, people performed 12 lineups.In half of those lineups, one of the faces that had a distinctive feature was presented along with 5 new faces.In the other half of the lineups, all of the faces were new.Participants in the study had to select the face they had seen before or say “none” if they thought all of the faces were new.

For half of the lineups that contained familiar faces, the distinctive feature was removed from the face that had been seen before.For the other half of those lineups, the same distinctive feature was given to all of the faces in the lineup.

It turns out that adding the same distinctive feature to each of the faces led to more accurate performance than removing the distinctive feature.People correctly identified the face they had seen about 50% of the time when all of the faces had the distinctive feature, but they identified it correctly only about 30% of the time when the distinctive feature was removed from the target face.Finally, people were able to state when there was no familiar face present about 60% of the time, regardless of whether a familiar distinctive feature was added to all of the faces or not.

This study demonstrates the importance of doing scientific research on practical questions like eyewitness lineups.It is not obvious that law enforcement personnel would predict in advance that adding the same distinctive feature to all faces in a lineup would improve the reliability of eyewitness identification.Over the years, though, psychological research has led to a number of improvements to the process of running lineups.